Methods and systems for inductive energy management

Abstract

A controller configured to be coupled to an electric motor. The controller including a processor programmed to receive a signal indicating a stopping command of the electric motor, and control a current such that a capacitor coupled to the electric motor is not overcharged by regenerative energy when a stopping of the electric motor has commenced, wherein controlling the current includes one of the following: upon receiving the signal indicating the stopping command of the electric motor, ramping the current down below a threshold level, or upon receiving the signal indicating the stopping command of the electric motor, forcing the current to circulate in motor windings to prevent regeneration of energy in the capacitor.

What is claimed is: 
     
       1. A controller configured to coupled to an electric motor, the controller comprising:
 a rectifier configured to convert an alternating current (AC) input voltage received from a power source to a direct current (DC) link voltage; 
 a DC link capacitor coupled to an output of the rectifier and having a capacitance below 10 μF, the DC link capacitor configured to receive the DC link voltage, wherein the DC link voltage drops to a value of approximately zero during each cycle when the AC input voltage transits a value of zero; 
 an inverter coupled to an output of the DC link capacitor and comprising a plurality of switches configured to condition the DC link voltage to output an AC voltage for driving the electric motor; and 
 a processor coupled to the DC link capacitor and the inverter, said processor programmed to:
 receive a measurement of the DC link voltage from a voltage sensor coupled to the DC link capacitor; 
 issue a stopping command to stop the electric motor by opening the inverter switches each time the DC link voltage drops to the value of approximately zero; and 
 control current provided to the electric motor to prevent overcharging of the DC link capacitor beyond a maximum voltage rating of the DC link capacitor by regenerative energy when a stopping of the electric motor commences as the DC link voltage drops to the value of approximately zero, wherein controlling the current comprises one of the following: 
 when the DC link voltage has the value of approximately zero, ramping the current provided to the electric motor down below a threshold level before opening the inverter switches, the threshold level being a level at which an amplitude of the current measured by a current sensor on the electric motor must be below before the electric motor is turned off to prevent regenerated voltage from exceeding the maximum voltage rating of the DC link capacitor; or 
 causing a short-circuit condition in windings of the electric motor to force the current to circulate in the windings to prevent any regeneration of energy back to the DC link capacitor when the DC link voltage has the value of approximately zero. 
 
 
     
     
       2. A controller in accordance with  claim 1 , wherein ramping the current down below a threshold level comprises:
 receiving the amplitude of the current; 
 determining the threshold level; 
 comparing the amplitude of the current with a maximum level of current based on the determined threshold level; and 
 ramping the current down below the determined threshold level. 
 
     
     
       3. A controller in accordance with  claim 1 , wherein the processor is further programmed to send a signal requesting an inverter to be in an off-state upon receiving the signal indicating the stopping of the electric motor. 
     
     
       4. A controller in accordance with  claim 3 , wherein the processor is further programmed to, prior to sending the signal to the inverter, ramping the current down below the threshold level. 
     
     
       5. A system comprising:
 an electric motor; and 
 a motor drive circuit comprising: 
 a rectifier configured to convert an alternating current (AC) input voltage received from a power source to a direct current (DC) link voltage; 
 a DC link capacitor coupled to an output of the rectifier and having a capacitance below 10 μF, the DC link capacitor configured to receive the DC link voltage, wherein the DC link voltage drops to a value of approximately zero during each cycle when the AC input voltage transits a value of zero; 
 an inverter coupled to an output of the DC link capacitor and comprising a plurality of switches configured to condition the DC link voltage to output an AC voltage for driving the electric motor; and 
 a controller coupled to the electric motor, the DC link capacitor, and the inverter, the controller comprising a processor programmed to:
 receive a measurement of the DC link voltage from a voltage sensor coupled to the DC link capacitor; 
 issue a stopping command to stop the electric motor by opening the inverter switches each time the DC link voltage drops to the value of approximately zero; and 
 control current provided to the electric motor to prevent overcharging of the DC link capacitor beyond a maximum voltage rating of the C link capacitor by regenerative energy when stopping of the electric motor commences as the DC link voltage drops to the value of approximately zero, wherein controlling the current comprises one of the following:
 when the DC link voltage has the value of approximately zero, ramping the current provided to the electric motor down below a threshold level before opening the inverter switches, the threshold level being a level at which an amplitude of the current measured by a current sensor on the electric motor must be below before the electric motor is turned off to prevent regenerated voltage from exceeding the maximum voltage rating of the DC link capacitor; or 
 causing a short-circuit condition in windings of the electric motor to force the current to circulate in the windings to prevent any regeneration of energy in the DC link capacitor when the DC link voltage has the value of approximately zero. 
 
 
 
     
     
       6. A system in accordance with  claim 5 , wherein ramping the current down below a threshold level comprises:
 receiving the amplitude of the current; 
 determining the threshold level; 
 comparing the amplitude of the current with a maximum level of current based on the determined threshold level; and 
 ramping the current down below the determined threshold level. 
 
     
     
       7. A system in accordance with  claim 5 , wherein the processor is further programmed to send a signal requesting the inverter to be in an off-state upon receiving the signal indicating the stopping of the electric motor. 
     
     
       8. A system in accordance with  claim 7 , wherein the processor is further programmed to, prior to sending the signal to the inverter, ramping the current down below the threshold level. 
     
     
       9. A system in accordance with  claim 5 , further comprising a voltage clamping device coupled the capacitor, the voltage clamping device configured to protect the capacitor from being overcharged by regenerative energy, the voltage clamping device configured to absorb the regenerative energy if the regenerated voltage reaches a maximum threshold. 
     
     
       10. A system in accordance with  claim 9 , wherein the voltage clamping device is at least one of a metal oxide varistor, a transient voltage suppressor, and a gas discharge tube. 
     
     
       11. A method for controlling a current to prevent overcharging of a direct current (DC) link capacitor coupled to an electric motor by regenerative energy when a stopping of the electric motor has commenced by opening switches of an inverter coupled to the electric motor, wherein the capacitor has a capacitance below 10 μF/kW, the method comprising:
 converting, by a rectifier coupled to the DC link capacitor, an alternating current (AC) input voltage received from a power source to a DC link voltage, wherein the DC link voltage drops to a value of approximately zero during each cycle when the AC input voltage transits a value of zero; 
 receiving, by a controller coupled to the DC link capacitor, the inverter, and the electric motor, a measurement of the DC link voltage from a voltage sensor coupled to the DC link capacitor; 
 issuing a stopping command to stop the electric motor by opening the inverter switches each time the DC link voltage drops to the value of approximately zero; and 
 upon issuing the stopping command, controlling current provided to the electric motor to prevent overcharging of the DC link capacitor beyond a maximum voltage rating of the DC link capacitor by regenerative energy when a stopping of the electric motor commences as the DC link voltage drops to the value of approximately zero, wherein controlling the current comprises executing one of the following:
 when the DC link voltage has the value of approximately zero, ramping, by the controller, the current provided to the electric motor down below a threshold level before opening the inverter switches, the threshold level being a level at which an amplitude of the current measured by a current sensor on the electric motor must be below before the electric motor is turned off to prevent regenerated voltage from exceeding the maximum voltage rating of the DC link capacitor; or 
 causing a short-circuit condition in windings of the electric motor to force the current to circulate in the windings to prevent regeneration of any energy back to the DC link capacitor when the DC link voltage has the value of approximately zero. 
 
 
     
     
       12. A method in accordance with  claim 11 , wherein ramping the current down below a threshold level comprises:
 receiving the amplitude of the current; 
 determining the threshold level; 
 comparing the amplitude of the current with a maximum level of current based on the determined threshold level; and 
 ramping the current down below the determined threshold level. 
 
     
     
       13. A method in accordance with  claim 11 , further comprising requesting an inverter to be in an off-state upon receiving the request to stop the electric motor. 
     
     
       14. A method in accordance with  claim 13 , further comprising, prior to requesting the inverter to be in an off-state, ramping the current down below the threshold level. 
     
     
       15. A method in accordance with  claim 11 , wherein forcing the current to circulate in motor windings to prevent regeneration of energy in the capacitor is performed until the electric motor has stopped or until the current decays below a threshold.